Confocal microscopy is a technique for optical imaging. It uses a spatial pinhole in combination with lenses and a laser to detect that portion of the reflected laser light from the sample that lies within a narrow range of the focal plane of the lenses. The detector can only observe one small portion, or pixel, of the sample at once so images are taken from over the entire sample area to be measured at a rate of several times per second and then reconstructed into a composite image by a computer. By so doing, confocal microscopy enables operators to obtain increased image contrast, provide three dimensional images or both. This principle was expressed in U.S. Pat. No. 3,013,467 hereby totally incorporated by reference and has since been developed in numerous other publications and patents.
Because it enables users to obtain enhanced haze free three dimensional images, confocal microscopy in addition to life sciences, applications has proven particularly useful in the semiconductor industry where it has been applied to inspection of wafers and other semiconductor materials and devices.
Recently the use of confocal microscopy to study the wear characteristics of chemical mechanical polishing (CMP) pads has become important. More particularly the use of laser confocal microscopy and a transparent window to determine contact area and indirectly, the surface characteristics and topography of CMP pads and other materials has become known. Such usage is disclosed in “Application of Laser Scanning Confocal Microscopy to Surface Topography Measurement of CMP Polish Pads,” D. J. Stein, D. L. Hetherington, F. B. Kaufman, J. L. Cecchi (Feb. 22-23, 1997) hereby totally incorporated by reference, “Visualization and Measurement of Contact Area by Reflectivity,” E. Diaconescu and M. Glovnea, (October 2006) hereby totally incorporated by reference and “Measurement of CMP Pad Contact, Deformation and Flow Resistance: Advances in Microtexture Design and Process Predictability,” by G. P. Muldowney, C. L. Elmufdi, B. Jiang, R. Palaparthi (Mar. 6-8, 2007) hereby totally incorporated by reference. The matching refractive indices of the sapphire and pad material mean that pad contact areas will show no reflection whereas pad non contact areas show reflection and interference fringes, allowing a clear contrast and determination of absolute contact area between the pad material and the transparent window to be made.
In 2006 in a paper entitled A Novel Optical Technique to Measure Pad-Wafer Contact Area in Chemical Mechanical Polishing by Carolina L. Elmufdi, Gregory P. Muldowney hereby totally incorporated by reference, it was observed that “Confocal reflectance interference contrast microscopy (C-RICM) uses a single focal plane to image the pad-wafer contact interface. A sapphire cover slip is used to provide optical transparency and to match the refractive index of the pad.”
The main problem of the prior art has been, however, that to prepare CMP samples with specific surface areas has required that samples be prepared in specific shapes suitable for observation and measurement. The sample must be carefully cut to the precise shape of the transparent window not leaving any burs or other anomalies on the edge of the sample that might result in uneven support of part of the applied load and bias the contact area measurements over the rest of the sample.
Any variation in the load or more specifically the pressure on any part of the pad in general contact with the transparent window results in anomalous and erroneous results. It is essential to determine the contact area under a uniform load so such edge features must be eliminated by either preparing samples with regular edges (for testing new materials) or careful cutting and preparation of samples to eliminate edge anomalies and burs in the case of testing of used pads. This can be very time consuming and reduce both the accuracy of the results and the time efficiency of the operation.
In U.S. patent application Ser. No. 12/108,720, incorporated totally herein by reference, a sample holder for confocal microscopy of CMP pad samples cut or otherwise removed from either new or used CMP pads that maintains a uniform load and pressure over the part of the sample visible to the confocal microscope by placing the pad behind a transparent window and holding it against the said window by a means comprising upper transparent window retaining means having an offset adjacent the transparent window having the same or essentially the same refractive index as the pad material so that when the pad is held against the transparent window, the edges of the pad are outside the outer edge of the transparent window; lower pad retaining means to press the pad under a known/load against the transparent window, which lower pad retaining means has a size less than the size of the pad; spherical force transmitting means pressed against the lower pad retaining means; through a load cell to measure the load transferred to the sample through lower pad retaining means, the spherical force transmitting means, the force transfer means and load cell from the posterior structural housing of the sample holder which is forced together with the said window retaining means by a force generating means; and means to adjust the known load and a method for using the same were disclosed. This device and method overcame the problems of the prior art cited above in regard to the problems created by edge anomalies when observing pad window surface area contact at a given load.
However, the aforementioned invention did not teach any method of determining how much displacement of the pad surface, that is to say compression or decompression of the CMP polishing pad, occurred when different loads were applied and by extension how much the surface area contact changed with specific change in such displacement. Displacement here refers to the displacement of the capacitance probe and the force transmitting means upon which it is riding to and from the window retaining means as the load is increased and decreased and the pad compresses and decompresses respectively and the corresponding “displacement” of the anterior surface of the sample relative to the posterior surface. This factor can provide operators and manufacturers of CMP pads as well as manufacturers of CMP pad conditioners with valuable information concerning the nature of the CMP pad surface and the effect of CMP processes on it. In particular such information would provide better knowledge of the roughness of the pad and depth, elevation and width of surface features on the pad.